The present invention relates to a system for making continuous, real-time, non-destructive measurements of constituents in optically dense fluids for applications including, but not limited to, pharmaceutical, food, beverage, fuel, enzyme, and chemical manufacturing as well as other industrial processes. In particular, the present invention is well-suited for performing measurements in aqueous fluids, which is significant in a plurality of applications such as bioprocessing.
Access to continuous real-time data on conditions and constituents in fluid processing applications is highly desirable in a multitude of branches of industry and research. While robust sensors are widely available to measure parameters such as pH, dissolved oxygen (DO), temperature, pressure, etc. in-situ and in real-time, such sensors to measure quantities of chemical constituents and suspended solids in fluids in real-time and in a robust manner have remained elusive in the art. Furthermore, sensors that are capable of measuring constituent concentrations often do so in a destructive manner, have inadequate sampling frequencies, and often require manual sampling and off-line analysis.
Quantification of constituent concentrations during fluid processes is most commonly performed by manual sampling and off-line analysis. Techniques such as high performance liquid chromatography (HPLC), gas chromatography-mass spectroscopy (GCMS), as well as various enzyme- and reagent-based electrochemical approaches are available for off-line constituent quantification. Such techniques may be accurate, but suffer in that they are destructive to the sample, often require expensive consumables, take long times to complete, and are prone to calibration difficulties. In addition, the hardware required to perform these analyses is commonly expensive to maintain and typically demand highly trained and dedicated personnel. Due to the labor requirements of manual sampling and time required to perform a measurement, the sampling frequency is typically insufficient to enable any meaningful feedback control of a process. In order to reduce the labor cost of manual sample analysis, samples are commonly run in batches and after a process has run to completion. In this case, constituent quantification data are of limited value as control strategies cannot be implemented and corrective action cannot be taken during a process.
Sampling systems are available to automate the sampling process with some of these measurement approaches, and feedback control capabilities have been advertised. However, the aforementioned limitations still apply, and the total sample and priming volume are often on the order of 1-10 mL, making these systems inapplicable to miniature- and micro-sized fluid processing vessels which are increasing in popularity in research and development environments.
Measurement of characteristics of fluids by optical means is well-known in the art. Electromagnetic radiation impinging on a medium may interact with the medium by absorption, scattering, or fluorescence. Measurement of transmitted, reflected, scattered, or fluoresced radiation by an appropriate sensor may be used to determine multiple characteristics or properties of the medium, such as concentrations of constituents or turbidity, simultaneously. Typical techniques include visible spectroscopy, infrared spectroscopy, and optical scattering measurements such as Raman spectroscopy. On-line constituent monitors utilizing Raman spectroscopy have been described in the prior art and scientific literature, however commercial examples of such systems are most commonly sold as generalized measurement platforms requiring a highly skilled operator to produce useful output. Fundamental issues such as fluorescence background and weak signal often limit the performance of Raman analyzers—while the Raman signal increases with the fourth power of the excitation laser frequency, background fluorescence increases proportionally with the laser frequency. Use of longer wavelength laser sources reduces the background fluorescence, however higher powers must often be applied to the sample to produce acceptable signal levels, and sample damage may result.
Analytical instruments utilizing the near-infrared (NIR) spectral range (roughly 700 to 3000 nm) have found application in numerous industrial and laboratory applications such as chemical processing, food and beverage manufacturing, petroleum processing, and pharmaceutical manufacturing. A variety of configurations of NIR spectrometers and analyzers have been demonstrated in the prior art, having the intended function of determining one or more characteristics or parameters of a substance or its constituents. All such instruments execute similar fundamental operations: generate NIR radiation; direct said radiation to a sample; resolve the resultant radiation having interacted with the sample spectrally; measure the resolved radiation with an appropriate sensor; predict or measure characteristics of the sample or its constituents. These operations need not be performed in the specified order here. For example, a tunable light source may be employed rather than spectrally resolving broad-band radiation into multiple smaller bands. Separation of the radiation into discrete bands may also be performed before interaction with the sample takes place.
Various methods may be employed to perform the fundamental spectroscopic operations. Light sources may include light emitting diodes (LEDs), tungsten halogen lamps, micro electromechanical systems (MEMS)-based sources, and infrared lasers. Spectral resolution of the optical energy may be performed by grating-based solutions (scanning or fixed); Fourier transform infrared (FTIR) interferometry; interference filtering; and acousto-optical tunable filters (AOTF). The sample may be interrogated by means of optical transmission, transflection, reflection, scattering, or fluorescence. The sample may be flowing or static. A variety of sample interrogation methods exist. Furthermore, both free-space and fiber-coupled optical delivery and collection means are possible. Detection of NIR radiation may be accomplished by means of single element or array sensors. Semiconductor sensors comprising, for example: silicon; germanium; indium arsenide (InAs); indium gallium arsenide (InGaAs); indium antimonide (InSb); and lead sulfide (PbS) are available. Other detection platforms including photomultiplier tubes (PMT) and thermal sensors are also available. Determination of sample characteristics, parameters, and constituent concentrations may be performed by one or more chemometric approaches.
In most implementations in the prior art, resolution of broad-band NIR radiation is performed by grating- and FTIR-based approaches. While these approaches may offer advantages in throughput and/or resolution, they are often insufficiently robust for deployment in industrial or manufacturing settings. For example, thermal and mechanical disturbances in industrial environments combined with the inherent mechanical sensitivity of these spectroscopic approaches often limits practical relevance in industry. Furthermore, FTIR instrumentation is often bulky and not accommodating to miniaturization, making such platforms difficult to deploy in laboratory and industrial environments where space is at a premium. Another common limitation of prior art NIR instrumentation when applied to fluid analysis is the speed over which such instruments scan over the wavelength range of interest. Such instruments commonly provide scan times ranging from several hundred milliseconds to several seconds. In applications with flowing or agitated fluid, gas bubbles and solids suspended within the fluid often traverse the optical beam used to interrogate the fluid. If the transit time of such bubbles or suspended solids is faster than the scan time, detection of such disturbances and minimization of the effects they impart on a measurement becomes difficult or impossible.
Optical emitters, elements, and in particular sensors utilized in prior art NIR analyzers are often optimized for spectroscopy in the first C—H overtone region. This also roughly overlaps with the optical telecommunications wavelengths in the 1.3 to 1.6 μm spectral band. Optics and optical emitters in this spectral region are well-developed and widely available due to ubiquitous use in the telecommunications industry. Systems optimized for spectroscopy in the combination band of 1.6 to 2.6 μm are less common. This wavelength band is of particular relevance for measurements of constituent measurements in aqueous fluids due to strong optical absorption features of the constituents as well as the favorable optical transmission of water. Liquid water presents strong optical absorption bands in the infrared due to fundamental O—H stretching vibrations. While strong absorption peaks exist near 3450 cm−1 (2.9 μm) and 5128 cm−1 (1.95 μm), the combination region therebetween is sufficiently optically transparent to enable transmission measurements to be performed in aqueous solutions.
Sampling approaches in common NIR analyzers include external and internal flow cells, cuvettes, vials, and fiber optically coupled probes. Performance from external flow cells and fiber optic probes tends to suffer due to significant NIR absorption in common optical fibers as well as low throughput owing to the large emitting area of NIR optical emitters compared to optical fiber apertures. Sampling approaches internal to the instrument often present connection and sample loop sterility challenges. Permanent hardware implementations of flow cells are common, though not particularly amenable to the cleaning and sterilization requirements of biotechnology industries.
Another significant limitation of prior art NIR analyzers is that they are most commonly developed as generalized spectroscopic instruments intended for use in a wide variety of applications. While the apparent flexibility may appeal to some users, these platforms often require a highly skilled operator to generate useful output from the instrument and data analysis software.
To overcome the limitations of prior art NIR analyzers, a platform is desired which: is robust to the environment in industrial facilities; is compact and easily portable; interrogates samples non-destructively, continuously, and in real-time; determines fluid component concentrations continuously within complex, dynamic matrices; accommodates samples that are highly turbid; provides a sterilizable sampling interface that may be disposable; enables process control; remains within specified operating parameters for extended periods of time; has a low maintenance burden; is able to reject spectra contaminated by gas bubbles within the fluid; is able to robustly eliminate outlier spectra due to short-term instrument drift or sporadic events within the process fluid; is readily calibrated for complex and/or dynamic sample conditions; maintains calibration over an extended time period; and does not require a highly skilled operator. Preferably systems may be provided which address as many of the aforementioned limitations as possible.